Trials with HM Submarine Seraph

British Preparations to Defeat the Type XXI U-Boat, September – October, 1944

By Lieutenant Commander Malcolm Llewellyn-Jones, MBE, MA, RN (Ret'd)

Introduction.

In 1944 the Royal Navy succeeded in modifying one of its conventional submarines, Seraph, to match the performance of the radically new Type XXI fast boats that intelligence showed Germany was developing.1 If the enemy had succeeded in getting the Type XXI to sea in large numbers, they would have revolutionised submarine warfare, and severely tested Allied anti-submarine defences. Senior officers warned of such a doomsday scenario at the time, and their bleak predictions have been selectively repeated in much of the subsequent literature.2 By contrast the present paper argues that the Admiralty moved more quickly and effectively to meet the incipient danger than has been generally appreciated. The swift conversion of Seraph, and the important sea trails immediately undertaken with her for the preparation of counter-measures, shows how supple and creative an agency the Admiralty had become in meeting the challenges of high technology.

Type XXI U-boat, U-2518, in Dun Laoghaire Harbour, 9 February 1946. This photograph was taken of the captured U-2518 and was kindly supplied by Lieutenant Commander Miles Chapman R.N. (Retd), whose wife took the picture. U-2518 was enroute to Cherbourg (Operation "Thankful", February 1946) on loan to the French Navy. For details of this operation see: Report of Operation "Thankful", Commander-in-Chief Plymouth, M.1849/1946, 9 March 1946, ADM 1/19510.

The British Identify the Type XXI U-boat Threat.

By mid-1942, the ubiquity and increasing effectiveness of Allied anti-submarine aircraft had forced Dönitz to contemplate the need for submerged U-boat operations. One possible choice was new submarine types powered by the Walter turbine using hydrogen-peroxide fuel. While this system promised high underwater speed, the U-boat's endurance would be limited and technical difficulties in controlling the highly volatile fuel ultimately precluded an operational system entering service during the war. By March 1943 the Germans, faced with insurmountable delays, conceived an alternative, hybrid U-boat combining the Walter-boat's streamlined hull with a powerful, but conventional, diesel-electric propulsion train, to attain high speed. Relatively long endurance would be achieved from a very large battery, capable of being recharged submerged by using a schnorkel. This was known as Type XXI U-boat.3

During the winter of 1943-44 the Admiralty's Naval Intelligence Department (N.I.D.) became aware of the enemy's developments, though the intelligence remained imprecise. Ultra provided few clues and interrogations of prisoners-of-war produced little substantive evidence, while continuously poor weather over Germany rendered Photographic Reconnaissance impotent. The breakthrough came in the spring when a new U-boat type was identified and credited it with an extreme diving depth, long endurance and high underwater speed4. In many ways the obscurity of the intelligence made little difference, for the British already understood the submarine design parameters required for high underwater speed. So, when the Captain Prichard, Director of Anti-Submarine Warfare (DA/SW), formed a Panel to investigate the problem it quickly to identified the necessary antidotes, issued in a paper by Prichard on 14 April 1944. He concluded that a U-boat capable of high speed and long endurance could not only dive further out from the convoy but it could also make several submerged approaches from any direction, making it essential to provide all-round convoy protection and with air escort patrols operating at greater distances5.

Though the Type XXI would probably attack at slow speed, when she could use her listening gear without interference from her own propeller noise, she would use high speed to evade escort hunts. The Type XXI's manoeuvrability would pose a serious difficulties for a single escort trying to hold contact, so the assistance of consort thought essential. Moreover, escorts would be handicapped when attacking. Depth-charge attacks would be rendered wholly ineffective if the U-boat got any warning of the attack. Escorts fitted with ahead thrown weapons, such as Hedgehog and Squid, would fare better, although with the relatively short, fixed firing range at which these weapons could be fired, combined with the rapid changes in bearing as a high-speed U-boat evaded, accurate attacks difficult to achieve. Single salvo attacks were unlikely to be effective and engagements would become asdic-controlled bombardments. For the future, Prichard thought, weapons would be needed that could fire at varying ranges as well as at any bearing relative to the escort. He even suggested the use of an acoustic, anti-submarine homing torpedo, fired from surface and air escorts, such as the American weapon code-named the "Mk 24 Mine", which was already in use6. To complicate matters further, the U-boat was likely to counter-attack with their Gnat anti-escort, homing torpedo. When approaching the U-boat, the escort would need to slow down below 7 knots so she would make insufficient noise to attract the Gnat, or, if capable, accelerate to 25 knots which was fast enough to outrun the Gnat. At the slower speed a fast U-boat could outrun the escort, while at the higher speed escort's asdic would be useless due to self-noise. The alternative anti-Gnat "step-aside" tactic would put the escort outside the Gnat's acquisition range, but would also allow a 12-knot U-boat the chance to escape. Alternatively, if the escort deployed its Foxer anti-Gnat noisemaker, then her asdic was degraded7. If all else failed, given that the U-boat's high-speed endurance was limited, perhaps the best tactic for the escorts was to maintain contact until the U-boat had to slow down, when it would become a relatively easy target.

During May 1944 the intelligence picture came into sharper focus and Prichard expressly warned the Captain, HM Anti-Submarine Experimental Establishment (HMA/SEE), Fairlie, of the existence of a U-boat with a streamlined appearance that was assessed to be capable of 16 knots submerged. The Captain HMA/SEE, who was already working on the fast submarine problem, complained that the Fleet knew nothing of this threat but Prichard believed that current training should concentrate on the known menace of the schnorkel-fitted U-Boat. He added that information on the Type XXI would be disseminated in the Monthly Anti-Submarine Report8. There was also considerable worry at this time about a "W-boat", thought to be a small submersible using a Walter turbine and capable of speeds between 25 and 50 knots! If this threat had materialised it could have proved dangerous to the D-Day invasion shipping, and Captain Roberts, from the Western Approaches Tactical Unit, was asked to devise counter measure tactics. Yet amidst these more immediate concerns the Admiralty energetically pursued antidotes to the Type XXI9.

The Conversion of HM Submarine Seraph.

Charles Lillicrap, the Director of Naval Construction (D.N.C.), had been a member of DA/SW's U-Boat Panel, and while responding to a N.I.D. paper during the previous October had concluded that a British U Class submarine could be modified to achieve a submerged speed of 13 knots for about 20 minutes. This could be achieved by removing '…all the appendages required for fighting and operational purposes….' In other word, by "cleaning up" the hull; blanking the torpedo tube apertures; removing the gun; fitting a smaller, streamlined bridge; removing a periscope and the radar mast; up-rating the main motors and fitting a larger battery. When Dr. Goodeve, the Assistant Controller (Research and Development), called a meeting on 6 June 1944 to discuss the Type XXI threat, Lillicrap was thus able to report on the performance that could be achieved from a converted British submarine acting as a fast target. Although the figures for a U Class were given at the meeting, the conversion of an S Class submarine had also been studied, which avoided the complicated modifications needed to the hydroplanes of a U Class. As a result the meeting recommended that an S Class be selected, which they felt sure could be converted in about two months to act as a fast underwater target for use in trials to measure the effectiveness of existing asdic gear and to develop new attack tactics10.

By coincidence, just three weeks before the Assistant Controller's meeting HM Submarine Seraph had been damaged during an inadvertent dive to some 500 feet while on patrol in the South-West Approaches. She was in Devonport Dockyard awaiting repairs and became the obvious candidate for conversion to the first of the "Slippery" S Class of streamlined fast underwater target submarines. At the regular Deputy Controller's Meeting on Tuesday, 13 June 1944 it was confirmed that the modifications for Seraph were likely to be approved and that work on the motors was to start along with tank experiments on the hydroplanes. The detailed plans were not submitted to the Deputy Controller until 23 June. Nevertheless, anticipating approval, Seraph had been taken in hand on 16 June by Devonport Dockyard for damage repairs and conversion to a fast underwater target with completion planned for the end of August 194411. The modifications included the uprating of the main motors from 1,400 to 1,600 shaft horsepower at full power, the fitting of coarser pitch T Class propellers that allowed the extra power to be converted to thrust, and the installation of a high capacity battery to extend her endurance. In addition, to reduce her drag Seraph was streamlined by fairing off apertures, such as, anchor holes, torpedo tubes and one-third of the free flood holes, as well as reducing the size of her hydroplanes. The forward planes were also fixed in the "out" position and given a more powerful control mechanism. The forward periscope, radar mast, Oerlikon anti-aircraft guns and the 3" deck gun were removed, and finally the profile of the conning tower was reduced. The last two measures contributed almost half of the 55% reduction in Seraph's drag, so that at 9 knots the shaft horsepower needed to propel her was cut from about 1300 to a little over 70012.

The "Fairlie" Asdic Trials.

On 23 September 1944 Seraph started her submerged speed tests at Loch Long as part of the "First of Class" trials, under the supervision of Constructor Commander Newton13. She achieved a mean speed of 12½ knots at periscope depth, compared to her "unconverted" maximum speed of 8¾ knots, though this was still 3½ knots slower than the Type XXI. She could travel even faster at greater depths, but was only able to maintain these speeds for about 15 minutes. However, it was the increase in her submerged endurance at lower speeds that was most spectacular. For example, Seraph could now maintain 12 knots for about 45 minutes, or 10 knots for just over 2 hours, and she could make 6 knots for 8 hours, roughly double her endurance before the conversion. Moreover, her manoeuvrability was enhanced at higher speeds, even though the control surfaces had been reduced in area. So, while Seraph's submerged turning circle was about 335 yards at 4 knots, it only marginally increased to 360 yards at 12 knots. By comparison the larger Type XXI was later assessed to have a turning circle of 420 yards at 12 knots14.

By 29 September, Seraph was ready for asdic trials in Inchmarnock Water, in the Firth of Clyde, where she embarked Professor McCrea from the Admiralty's Directorate of Naval Operational Research (DNOR) and, perhaps, J. A. Hakes, a scientist from the nearby HM Anti-Submarine Experimental Establishment (HMA/SEE), Fairlie15. They had to contend with high westerly winds, which caused very broken water conditions until the last two days when the weather moderated. Readings of temperature versus depth were taken throughout the trial and, although none exhibited extreme temperature gradients, asdic ranges were extremely changeable, which the trial team attributed to the changes in the sea state and the effects of operating in enclosed waters. The trial team had until 6 October to evaluate the detection ranges and characteristics of the echo and hydrophone effect, known as H.E16. In the short time available Seraph made twenty runs on steady course at either periscope depth or 150 feet and usually at either 4 or 12 knots. The old corvette HMS Kingfisher was used to gather data by measuring the strength of the asdic echoes and H.E. against a calibration signal injected into her Type 128 asdic set. Kingfisher opened and closed the range, while trying to maintain the same inclination relative to the submarine, though Seraph's short endurance at high speed made it impossible to gather asdic data for all submarine aspects during a single run17. Asdic ranges on the target's beam were obtained during the poor conditions at the beginning of the trial, while most of the data at fine inclinations within 15o of the bow or stern was gathered towards the end, when asdic ranges were particularly good. This made the correlation of the results problematic.

McCrea had supposed that the streamlining of Seraph would produce weaker echo returns but beam-on she had an asdic echo equivalent to that for any submarine of her size. He had also forecast that ranges might '…fluctuate up and down for a progressive change of inclination.'18 At fine inclinations to the submarine McCrea's impression was that the ranges were reduced by as much as a half, compared to those on the beam. Hakes' later detailed analysis confirmed McCrea's interpretation. Hakes warned, however, that the variability of the water conditions and the short duration of the trial meant that the results only gave a rough indication of the echo strengths. Hakes' examination showed that under average conditions the echo level was always higher than the target's H.E., so the detection range was determined by the level of background noise at the asdic receiver, which, of course, increased with the ship's speed.

The analysis showed that the maximum range for echoes to record on the asdic was just over 4,000 yards with Kingfisher proceeding at a quiet speed, that is, up to 16 knots. But as her speed rose to 20 knots, the ship's own noise began to interfere with the asdic, and the maximum echo range decreased to 2,600 yards. Yet as the water conditions deteriorated Seraph's H.E. became dominant, except at very short ranges, because the asdic pulse, having travelled out from the asdic transmitter to the target and back to the receiver, lost acoustic power on both outward and inward paths, whereas the H.E., was only degraded during its single path from the submarine to the asdic receiver. With Seraph at 12 knots in these conditions, the submarine's H.E. masked the echoes to such an extent that they could not be recorded at ranges greater than 800 yards. Hakes observed, however, that in any water conditions, echoes '…should always be heard against H.E. noise from the submarine.'19 Ultimately, as the ship's speed increased to about 22 knots, so the noise of the water flow around the asdic and her own propeller eventually drowned any chance of echo detection.20 No runs were carried out with Foxers streamed, though Hakes was able to deduce, somewhat optimistically, that under average conditions echoes might be detected aurally out to 1,600 yards, though they would not paint on the recorder beyond 1,300 yards. Under poor asdic conditions these ranges were expected to reduce 700 yards and 500 yards.

Although Seraph presented a poor echo at fine inclinations, when she was travelling at speed towards or away from the escort the received echo exhibited an apparent change in pitch, known as the doppler effect. This was very marked and easily noticeable to the listening asdic operators and allowed them to distinguish the echo from the background clutter and the submarine's own wake. At long range where the echo was weak and not always visible on the recorder, the doppler could easily be detected by ear. On one occasion the submarine at 12 knots had been held by echo at 1,700 yards on the beam, but only 750 yards stern-on, whereas her doppler was heard out to 2,700 yards. Although at 12 knots Seraph was considerably less noisy than a normal S Class at its maximum speed of just over 8 knots, her H.E. could still swamp the asdic's receiver circuits and mask the echo, which eliminated the advantages of the doppler. McCrea noted that this problem was already being studied.21 On the other hand, when Seraph was at 6 knots, or less, Kingfisher could not detect any H.E. from the submarine; even at ranges as short as 500 yards. But, as the submarine's speed increased there was a sudden rise in the H.E. level, reaching a peak at 9 knots and then remaining at this level as the speed increased to 12 knots.22 The noise was uniform at all depths and on all bearings, except astern, where the observations were too few to draw any conclusions.23 At 12 knots Seraph's H.E. was always heard by Kingfisher on a listening asdic set, though not always if the gear was transmitting. The maximum range was about 5,500 yards with the ship stopped, though as Kingfisher increased speed to 16 knots the H.E. range decreased to 2,000 yards and, Hakes noted, Kingfisher could obtain bearings of the H.E. at moderate ranges.

The British Analyse Type XXI U-boat Operations.

In September 1944 Captain Howard-Johnston, the Director of the Admiralty's Anti-U-Boat Division (DAUD), included a provisional appreciation of the Type XXI U-boat in the Monthly Anti-Submarine Report. The information undoubtedly drew on the detailed paper prepared by Professor E. J. Williams, of DNOR, which was in turn was probably based on Ultra information received in N.I.D. by 23 August. This revealed the Type XXI to be about 245 feet in length, with a displacement of 1,600 tons, a submerged speed of 17 knots, long endurance from its abnormal battery capacity.24 The data allowed the earlier forecast of May to be refined. The potential tactics needed to counter the Type XXI were analysed and exercised on the tactical table at the Western Approaches Tactical Unit (WATU) in Liverpool under the guidance of Captain Gilbert Roberts. Williams and Howard-Johnston agreed that the Type XXI might be operated either in packs in mid-Atlantic, or independently in focal or inshore areas. Pack tactics were essential if the enemy were to achieve significant opportunities to attack, while mid-ocean operations would expose the submarines to the lowest air threat. Existing schnorkel-fitted U-boats would not achieve better results in pack operations, for though they had immunity from air attack it was at the expense of their tactical mobility, but with the Type XXI, Howard-Johnston warned, the situation would be very different. In reasonable sea conditions by using alternate periods of snorting and running on batteries the Type XXI could make a steady 8 knots dived without net depletion of the battery. In theory this speed was fast enough to keep up with a convoy, although U-boat control, essential for effective pack tactics, would be hampered by prolonged submerged operations. When far away from its quarry, however, even the Type XXI would have to spend some 35% of the time on the surface, mainly at night, to charge batteries (this method being more efficient than schnorkelling) and in order to make the higher transit speeds necessary to intercept a convoy. This was only half the time needed by the older types and the Type XXI would thus be much less vulnerable to air attack.

Again, compared to the older schnorkel-fitted U-boat, the Type XXI's higher speed would theoretically make it about 5 times less likely to be detected by asdic search if the escorts were delayed in arriving at a datum.25 However, when travelling at high speed the U-boat would be partially deaf as a result of its own noise and, it was believed, might also provide a better asdic target, perhaps because of the doppler effect. It was also assumed that the H.E. detection range against a 12 knot submarine would be considerably greater than the asdic range, and would increase with target speed, but would decrease rapidly as the U-boat went deeper, because the greater pressure would suppress the propeller's cavitation. Still, there was a good chance that the submarine's H.E. might give the escorts a greater chance of detection, than if they relied solely on asdic. However, if the surface forces were able to get to the datum position reasonably quickly, then they would be able to cover the probability area on their first sweep, even if the submarine was doing, say, 12 knots. If the Type XXI was to slow down to carry out listening searches, it could then manoeuvre to avoid approaching surface forces. William's observation prophesied the tactics employed by U-2511; the only Type XXI to encounter Allied anti-submarine forces as the war came to a close. Her captain Lieutenant Commander Schnee by making a 30o alteration of course and, remaining submerged, avoided the escort group.26

As had been discovered during simulations on the Western Approaches Tactical Unit tactical table, that if a Type XXI was detected it could use high submerged speed and rapid acceleration to prevent an escorts settling in on the asdic echo, unless it used new H.E. tracking tactics. Ahead thrown weapons would still have a relative advantage when attacking a high-speed U-boat and overall Williams remained optimistic, especially as the Germans might be devoid of the high standards of training needed to exploit the Type XXI after their heavy defeats during 1943. He believed, that the threat could be countered given escort training programmes against fast submarine targets, the use of night capable shore-based Very Long Range and carrier-based aircraft in mid-Atlantic, and the development of sono buoy tactics to search a datum or to detect a fast U-boat trying to follow a convoy. Much of this work was already in hand and once discussed by the Commanders-in-Chief at Western Approaches and Coastal Command, needed to be reviewed at the Cabinet Anti-U-Boat Warfare Committee.27

The Tactical Trials with the 19th Escort Group and Seraph.

On 3rd October 1944, mid-way through Seraph's trials at Fairlie, a high-level meeting was held at Commander-in-Chief, Western Approaches Headquarters in Liverpool, chaired by Captain Donald Macintyre, Admiral Horton's Chief-of-Staff. The meeting included representatives of Commander US Forces in Europe, RAF Coastal Command and Naval Aviation, Flag Officer (Submarines), the Directors of the Anti-U-boat, Anti-Submarine Warfare, Naval Intelligence, and Naval Operational Research Divisions, as well as the training and experimental establishments. Horton opened the meeting by focussing on the threat posed by the German inshore campaign and on the crucial need to strengthen the fighting standards by training, especially against training submarines fitted with a dummy schnorkel.28 The meeting found time to consider the tactical trials programme for Seraph, and agreed that she would exercise in the Irish Sea "Rockabill" Sanctuary with the experienced 19th Escort Group, consisting of HM Ships Hesperus (the Senior Officer), Goodall, Anguilla and Bullen, with the addition of Black Swan and, later, Loch Fada. The intention was for the naval exercises to be followed by trials with aircraft of 15 Group, Coastal Command, and the Fleet Air Arm, though these did not start until November. It seems likely that the detailed naval programme was orchestrated by Commander M. J. Evans, Training Commander, Western Approaches, who was nominated by Howard-Johnston to conduct anti-schnorkel trials being run at the same time.29 Professor McCrea was embarked in HMS Black Swan and, although he had to leave before the trials were completed, he discussed the results with the Training Commander.30

The tactical serials with Seraph started on 10 October and ran through to 30 October 1944. The primary objective was to develop effective tactics against a Type XXI U-Boat detected in the close vicinity of a convoy.31 The planning assumed, as Professor Williams had suggested, that the Type XXI might approach a convoy on the surface, then diving and using high speed to penetrate the screen, or, subsequently, to evade counter attacks by the escort. Thus, Seraph, was to dive three miles ahead of the convoy, simulated by Anguilla, Bullen and Black Swan, while the close escort, formed by Hesperus and Goodall, was to respond by the escort nearest to the diving position executing an immediate reaction "Delta" search to gain asdic contact, while the second ship moved to cover the front of the convoy and subsequently to carry out a "double Observant" square search starting four miles out from Seraph's diving position.32 Once the escorts achieved a dummy attack the submarine was to surface.

In the event these serials proved unworkable, because Seraph's endurance at high-speed was so short that by the time the escorts gained asdic contact the submarine was unable to continue at high speed for long enough for the attack to be made, and the point of the trial was lost! It was clear that greater benefit could be derived from using a tactical table, such as the one at the Western Approaches Tactical Unit, once the detection ranges had been measured. It was probably during these early serials that the prohibitively hampering effects of the Foxer, anti-Gnat towed noisemakers were discovered. Even when the water conditions were particularly good and detection should have been certain, the interference was so severe that the chance of detecting an echo was greatly reduced, while the likelihood of distinguishing H.E. from the background noises was practically nil. As a result the remainder of the trials were conducted without using Foxers, with the idea that if adequate methods could be found without their use, then a type of noisemaker could be developed that interfered least with the new tactics.33

Sketch illustrating the combined use of the "Delta" and "Observant search plans (above) as proposed during trials with Seraph in 1944

With the attempt to investigate approach tactics relegated to the tactical table, the bulk of the trials with the 19th Escort Group were used to explore the problems involved in attacking a fast U-boat, principally with ahead thrown weapons. In particular, the trial team wanted to see if the Type 144 asdic-fitted escorts could aim off the instantaneous target bearing at the moment of firing, thus allowing for the submarine's movement during the flight of the anti-submarine projectiles. This "throw-off", along with the timing of the correct moment to fire, could be calculated automatically by the asdic, provided an echo could be maintained on the asdic recorder trace. Of course, the weaker echoes at fine inclinations caused by Seraph's streamlining could still be distinguished at longer range from the doppler effect, at least aurally, though they could also be masked by the submarine's H.E. It was noticed that contact was often lost as Seraph accelerated, probably because the cavitation increased from her propellers, causing the louder H.E. to drown the asdic echo. There were times, McCrea noted, when the H.E. not only masked echoes but induced the asdic receiver to suppress the H.E. signal itself, both in the operator's headphones and on the recorder. Adopting an intermittent listening watch during the asdic search could circumvent the problem, though McCrea wondered if there were technical modifications that could solve the difficulty.34

To explore how these factors would combine, the tactical trials were planned to pose increasingly complex attack settings for the anti-submarine escorts, starting with single-ship attacks against Seraph at slow speed and on a steady course, and progressively escalating to two-ship hunts with the submarine allowed complete freedom of evasion in both course and speed. The Type 144 Asdic, in common with all contemporary sets, transmitted a fine "searchlight" beam. The standard operating procedure was to step the asdic beam over small bearing increments to determine the "cut-offs" of the target's extent, and thereby to measure the bearing of the submarine. This procedure left little margin of error when holding contact on a high-speed U-boat, which could easily elude the narrow asdic beam, especially at the short ranges typical of these dynamic engagements. Moreover, by the time the single escort had carried out a wider search, the submarine could have escaped outside the asdic's detection range. In consequence a single ship '…could easily be thrown off the scent.'35 The solution lay in exploiting the bearing of the submarine's H.E. and using two-ship hunting teams.

H.E. could be used to detect the submarine at 4-5,000 yards. These ranges seemed unaffected by Seraph's depth, at least down to 200 feet, and were also less sensitive to variations in water conditions that caused extreme fluctuations in the echo ranges. These results were not consistent with earlier work by Directorate of Scientific Research and the Admiralty Research Laboratory, at Teddington, which had suggested that the H.E. level would fall off as the submarine's depth increased. McCrea wondered whether this would be a factor in dealing with the Type XXI, which he assumed could dive to great depths.36 Given reasonable H.E. ranges, however, two escorts were able to locate a fast submarine by plotting the intersection of H.E. bearings taken simultaneously from each ship. The procedure relied heavily upon the rapid passage of asdic information between the co-operating escorts and, crucially, on each ship keeping an up to date tactical chart. Such expertise only came with training and practice, not only for the asdic teams but also of all the personnel involved in inter-ship communications. When teams were worked up, Howard-Johnston noted that good fixes on the submarine were possible. In this way, he considered a fast U-boat was unlikely to escape from a practiced two-ship group, unless the asdic conditions were exceptionally poor. Black Swan and Goodall amply illustrated this. Once they gained experience with a fast submarine, they achieved almost continuous contact and 5 attacks during a two-hour trial, even though Seraph was at 9 knots for nearly 40% of the time. However, McCrea, alluding to the forecast speed of the Type XXI U-boat, worried that if a submarine could achieve just a few knots greater speed, the dynamic tactical balance could tilt in favour of the submarine, perhaps making the problem insurmountable.37

The results against Seraph confirmed that attacks were possible against a submarine on a steady course at 12 knots, though there were many practical difficulties as well as random errors that would cause a reduction in their accuracy. Commander Evans noted that the ships' anti-submarine teams had achieved good attacks when they visited HMA/SEE's attack-teacher ashore. Attacks remained practicable, even when Seraph took a modest avoiding action, provided a recordable echo was received. Unfortunately, the short time available during the trials did not permit the trial team to measure the statistical accuracy of these attacks and to compare them with those against slow U-boats. Howard-Johnston, quoting Commander Evans, warned that "…the difficulty in attacking [was] primarily due, not to the unsuitability of ships or instruments…but to the very reduced margin or error which her high speed permits the hunting ships."38

Although a reliable forecast of likely German tactics was needed, McCrea thought that enough data was available from Seraph's trials for planning to start on tactical counter measures to the Type XXI. For example, given the limited contact holding capability of a single ship, McCrea suggested that any escort should immediately join a ship gaining a contact, even if of doubtful quality. New asdic procedures were also needed to exploit the detection opportunities offered by H.E. However, even a fast U-boat could to be encountered at slow speed, when it would only be detected by echo. The resultant "combined echo and listening sweep" consisted of two successive asdic sweeps from 80o left to 80o right of the ship's head, taking about 6 minutes, followed by two all-round H.E. sweeps, lasting about 40 seconds. Compared to dedicated echo or H.E. sweeps, this combined sweep marginally reduced asdic search efficiency, though the area swept for H.E. was practically the same. The same tactics were employed during post-war trials designed to confirm the results obtained with Seraph and using U-2326, a Type XXIII coastal U-boat, and again used successfully by the Londonderry flotilla of the Joint Anti-Submarine School in exercises with the US Guppy Trumpet fish in 1947 and the French ex-Type XXI Roland Morillot in 1950.39

The Impact of the Trials with Seraph.

The streamlining of Seraph, somewhat to the surprise of the scientists, had not reduced the asdic echoes seen from the beam, though as expected there was a significant reduction in the echoes observed at fine inclinations. This was tactically more important, since this was the aspect on which most U-boats would be encountered and hunted. The Admiralty had accurately forecast the value of doppler and H.E. in detecting and holding a fast U-boat contact. The difficulties a single ship would have in trying to counter this threat had also been correctly foreseen, as had the dynamic problems of attacking fast U-boats using ahead thrown weapons. Though some British theoretical calculations of weapon effectiveness against a fast U-boat made gloomy reading, the trials with Seraph were by contrast reassuring for they showed that, although attacks against a fast target were difficult and were sometimes abandoned, regaining contact was made easier because of the H.E. and doppler. Moreover, at high speed even the Type XXI's endurance was limited and as she was three times the size of Seraph, the U-boat would produce a correspondingly larger echo.40

The trials confirmed the Admiralty's assessment of how to deal with a fast U-boat, at least one capable of 12 knots, though there remained some lingering doubts over the impact of the Type XXI's somewhat higher speed. Furthermore, there were questions that remained unanswered. For example, how many ships would be needed to maintain contact on a Type XXI evading at high speed until it was forced to slow down, and how many attacks would be needed to achieve a kill? How would these U-boats be located in the open ocean? Would it be the case, as with the conventional schnorkel-fitted U-boats, that contact would normally be achieved only after a torpedo attack? It was clear that there were no immediate technical solutions on the horizon and that the remedy to the Type XXI lay in erecting a series of hurdles, such as, bombing of their bases, mining of the work-up areas, intensive patrolling of transit routes as well as heavy protection of convoys (which induced the retention of many escorts in European waters that had been intended for operations in the Far East).41

Douglas McLean has shown that it took the Royal Navy some six months to master the U-boat schnorkel tactics. 'It must be emphasised,' he points out, 'that simply promulgating tactical procedures is only the first step in actually employing new methods; plans must be absorbed and practiced by all ships before they can be effectively employed, and in the best of conditions this takes a good deal of time.'42 This would have been true had the Type XXI threat materialised, and the Admiralty recognised the problems. Admiral Cunningham, the First Sea Lord, while confident that in time the Admiralty and Coastal Command could develop the countermeasures to deal with this new threat and, thereby, break a renewed offensive, he was '…extremely doubtful whether this is likely to be achieved in time to prevent serious losses to shipping.'43 To mitigate this problem Seraph was used from November 1944 onwards for 'special duty', a euphemism for the training of '…selected escort groups' in countering the Type XXI. So heavy were the demands, that by early 1945 Seraph was joined by two more converted S Class, Satyr and Sceptre. In the last months of the war there were also trials exploring the performance of sono buoys, which Professor McCrea thought might have worthwhile possibilities against a fast submarine. However, because of the frequencies at which it listened, their capabilities would rapidly reduce as the target's depth increased. Between 26 November 1944 and 13 April 1945 the Coastal Command's Air Sea Warfare Development Unit carried out eight days of trials against Seraph, using American AN/CRT-1A sono buoys. The results were not conclusive but showed a tendency for the ranges to increase with target speeds above 5 knots, and to decrease as the submarine went deeper.44

The whole episode with Seraph demonstrates the Admiralty's ability to rapidly and successfully co-ordinate the expertise available from operational, scientific and engineering departments. Their accumulated knowledge and skills were more important than the intelligence picture, even that from Ultra, which remained obscure until after the detailed planning for Seraph's conversion was well under way. Moreover, this conversion was directed by staffs who understood the modifications necessary to convert a conventional submarine into a fast target. It is noteworthy that the modifications to Seraph were started on 16 June just one month after the first Type XXI, U-2501, was launched, and that the first trials with Seraph's were completed some seven months before the first Type XXI operational cruise by U-2511.45 If encounters had occurred between anti-submarine forces and the Type XXIs then the trials and subsequent training exercises with Seraph would have given the British the optimism and confidence that they could have dealt with the fast U-boat threat, even if there had inevitably been a long period while new tactics were mastered amidst combat conditions. It is easy to magnify the (forecast) technical capabilities of the Type XXI, while ignoring British tactical remedies. When these are offset it is difficult to accept Captain Howard-Johnston's assessment, written 36 years after these events, that '…we might have lost out against the new enemy, the Type XXI….'46 We will never know for certain, for in the end the German fast U-boat threat did not materialise. Instead it re-emerged in the real, or perceived, menace from Soviet submarines exploiting the Type XXI technology…but that is another story.

Many accounts emphasise the 'menace' of the 'more real and more disturbing' threat of the Type XXI, to which there was 'no ready technical or operational answers.' See: Stephen Roskill, The War at Sea 1939-1945, Vol. III, Part II: The Offensive 1st June 1944-14th August 1945 (London: HMSO, 1961), p. 290; John Terraine, Business in Great Waters: The U-Boat Wars 1916-1945 (London: Leo Cooper, 1989), p. 662; and Correlli Barnett, Engage the Enemy More Closely: The Royal Navy in the Second World War (London: W. W. Norton & Co., 1991), p. 854. One important exception is Douglas M. McLean, 'Confronting Technological and Tactical Change: Allied Antisubmarine Warfare in the Last Year of the Battle of the Atlantic', Naval War College Review, Vol. 47, No. 1 (1994), pp. 87-104.

The Foxer was a technical antidote to the anti-escort German naval acoustic torpedo (or "Gnat"). The Foxer, or Pipe Noise Maker (P.N.M.), consisted of two solid rods, secured at the ends, but allowed to vibrate when towed through the water. The noise made was similar to, but louder than, the H.E. produced by an escort and was intended to seduce a Gnat away from the ship's propellers. The problem of approaching a fast U-boat was still being studied in early 1945. DNOR Report, 'Anti-Gnat Tactics against Fast U-Boats', H. L. Welsh, 21 February 1945, ADM 219/274.

'Monthly Log of HM Submarine "Seraph", Month of September 1944', ADM 173/18701. 'Asdic trials with HM S/M "Seraph" as target', J. A. Hakes, Research Note No. 53, HMA/SEE Fairlie, November 1944. Information Centre, Defence Evaluation and Research Agency [DERA] Winfrith, Accession No. [A.N.] 28144. Report No. 72/44, 'Notes on A/S Trials with a Fast Submarine', W. H. McCrea, 9 October 1944, ADM 219/154. Interview with Professor Sir William McCrea FRS, 17 April 1998. There is no direct evidence that Hakes embarked, though as author of one of the trial reports, it seems likely that he did.

Hydrophone Effect was principally due to the collapse of the bubbles formed by a rapidly spinning propeller, and sounded like a "hiss" to a listening asdic operator. There were other sources of H.E., such as to noise generated by the flow of water past the hull or from internal machinery. Robert J. Urick, Principles of Underwater Sound, 3rd edn. (California: Peninsula Publishing, 1983), p. 334.

Re-charging the large battery was a protracted business and was carried out alongside. 'Monthly Log of HM Submarine "Seraph", Month of October 1944', ADM 173/18702.

Report No. 72/44, 'Notes on A/S Trials with a Fast Submarine', W. H. McCrea, 9 October 1944, ADM 219/154. It is likely at this stage of scientific understanding that asdic echoes were thought to be returned from the outer surfaces of the submarine. In fact the sound also penetrated to the inner (pressure) hull and reflections could occur from "internal" structures. These additional reflected highlights probably contributed to the uneven echo pattern observed. Discussion with Mr. Peter Evans, DERA Bincleaves, Weymouth, 19 November 1998.

'Asdic trials with HM S/M "Seraph" as target', J. A. Hakes, Research Note No. 53, HMA/SEE Fairlie, November 1944. Information Centre, DERA Winfrith, A.N. 28144, p. 4. It is not entirely clear how these figures were determined, since Kingfisher is only credited with a maximum speed of 20 knots. H. T. Lenton, British and Empire Warships of the Second World War (London: Greenhill Books, 1998), p. 267.

Seraph's T Class propellers had to turn slower and so would be less prone to cavitation at low speeds than would have been the case with her original propellers. For example, Sahib (an unconverted S-class) on her first of class trials had required 406 propeller rpm to achieve 8.82 knots, whereas at 12½ knots Seraph needed only 411 rpm. D. C. R. Webb to author, 6 August 1997. For a submarine of this period cavitation would start at about 6 knots and, thereafter, the noise would increase dramatically with speed – typically a 100-fold increase in intensity over a few knots. Once cavitation was fully established, in this case at about 9 knots, then it would not increase much as speed was increased – barely doubling in intensity, per knot. Peter Evans, E-mail to author, 22 December 1998.

It had been found from HMS Graph (ex U-570) that her wake could mask her H.E. to such an extent that it could not be heard even at short range by an anti-submarine escort within 20o of the submarine's stern bearing. C.B. 4097 (2) (44). 'Conduct of Anti U-Boat Operations', Part 2: 'Detection and Action', Anti-Submarine Warfare Division, November 1944, ADM 239/298, Paragraph 188.

The area to be searched by the escorts would grow exponentially with the square of the U-boat's speed, so it would be much larger for the Type XXI. DNOR Paper, 'Type XXI U-boat' with an Appendix 'Type XXI U-boat (A Provisional Appreciation)', E J Williams, 4 September 1944, ADM 219/150.

'Minutes of a Meeting held at ACHQ Liverpool on 3rd October 1944 to discuss a Programme of Schnorkel and other Trials and Practices', Admiral Horton, 4 October 1944, ADM 1/16121, p. 2.

Minute by C. D. Howard-Johnston, DAUD, 10 October 1944, ADM 1/16121. Evans was well qualified having commanded the escort during the battle around ONS.18 and ON.202 during which the U-boats used the Gnat anti-escort torpedo. Stephen Roskill, The War at Sea 1939-1945, Vol. III, Part I: The Offensive 1st June 1943-31st May 1944 (London: HMSO, 1960), pp. 38-40.

Regrettably, the Training Commander's report has not survived, though extracts were reproduced by Howard-Johnston in the Monthly Anti-Submarine Report. for November 1944.

"Observant" was the code word for a particular type of square search, in use from at least February 1943, used to cover the perimeter of the area thought to contain a U-boat. The sides of the square were initially two miles and were expanded at the rate of the U-boat's presumed movement. A "double Observant" started at twice the distance from the datum. 'Monthly Anti-Submarine Report, February 1943', 15 March 1943, ADM 199/2060. 'Monthly Anti-Submarine Report, February 1944', 15 March 1944, ADM 199/2061. 'Monthly Anti-Submarine Report, April 1944', 15 May 1944, ADM 199/2061. Doug McLean, E-mail to author, 'Plans "Delta" and "Observant", 2 March 1999. Lt Cdr Miles Chapman to author, 1 March 1999. Plan "Delta" was enshrined in Article 52 of the Admiralty Convoy Instructions for action when a U-boat was sighted in the vicinity of a convoy. 'Admiralty Convoy Instructions to Escorts (Short Title A.C.I.), 1944', Anti U-Boat Division, August 1944, ADM 239/345, p. 45. ACI's refers to C.B. 4097, 'Conduct of Anti-U-Boat Operations'. Regrettably the relevant section (Part 4) in the PRO copy is missing! C.B. 4097. 'Conduct of Anti-U-Boat Operations, 1940, Anti-Submarine Warfare Division, November 1940 (with amendments 27 September 1945), ADM 239/298. Plan "Delta" may be the same immediate reaction plan as that described in Admiral Adams Papers (now in the Churchill College Archive Centre, Cambridge) which consisted of a pre-defined series of search tracks followed by the escorts in the close vicinity of the datum position. It is not, however, clear how the "Delta" and "Observant" were co-ordinated.

Report No. 80/44, 'Notes on A/S trials with a fast submarine. ROCKABILL, Oct. 10-30, 1944', [W. H. McCrea], 11 November 1944, ADM 219/160, p. 1. "Publican", an expendable noisemaker, was pursued at high priority, in the hope that it would cause less interference to the asdic. Trials were also progressing with "Nightshirt", the use of bubble screens around propellers to allow ships to operate at higher speeds without danger from the Gnat, as well as improvements to the asdic to reduce the effect of the interference and allow higher operating speeds. Captain N. A. Prichard, DA/SW, to First Lord, 24 November 1944, ADM 205/36.

Cavitation, and hence H.E., would be expected to be suppressed by the water pressure as the submarine goes deeper, but there are anomalies and levels can increase with depth. Post-war trials with a Type XXI showed H.E. levels at 6 knots reducing significantly between 50 and 150ft, while at 10 knots they remained unchanged, and at 15 knots the H.E. level increased marginally. Robert J. Urick, Principles of Underwater Sound, 3rd edn. (California: Peninsula Publishing, 1983), p. 338.

At this time the Type XXI was assumed by DNOR to have a submerged speed of 17 knots, though the assessed speed was reduced to about 15 knots by DAUD. DNOR Paper, 'Type XXI U-boat' with an Appendix 'Type XXI U-boat (A Provisional Appreciation)', E J Williams, 4 September 1944, ADM 219/150 and 'Monthly Anti-Submarine Report, November 1944', ADM 199/2061, p. 14.

'Possibilities of the Coming [April 1945] U-boat Offensive', Fawcett, Naval Staff, Coastal Command, 1 December 1944, FWCT 2/4/5. This paper had been requested by Howard-Johnston, DAUD, from Commander Fawcett, which had served in DAUD before moving to Coastal Command. 'The RAF in Maritime War, Vol. V: The Atlantic and Home Waters, The Victorious Phase, June 1944 – May 1945', n.d., AIR 41/74, pp. 201-202.

Douglas M. McLean, 'Confronting Technological and Tactical Change: Allied Antisubmarine Warfare in the Last Year of the Battle of the Atlantic', Naval War College Review, Vol. 47, No. 1 (1994), p. 94.

'A Forecast of the Results of the U-boat Campaign during 1945', Memorandum by the First Sea Lord, 6 January 1945, PREM 3/414/1. Cunningham, not noted for his anti-submarine acumen, doubtless relied on his staff experts for such opinions.

C. D. Howard-Johnston to J. D. Brown, Naval Historical Branch, 24 February 1980, Captain C. D. Howard-Johnston Papers, HWJN, Churchill College Archive Centre, Cambridge. [The author is grateful to Carolyn Lye for passing a copy of this letter immediately the collection was opened to public view.]

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Lieutenant Commander Malcolm Llewellyn-Jones, MBE, MA, RN (Ret'd),

After twentysix years in the Fleet Air Arm, including an anti-submarine squadron command, Malcolm Llewellyn-Jones retired and completed an MA in War Studies. At the time of writing this article he was researching the anti-submarine revolution, 1944-1954, for a PhD at King's College, London.

The author gratefully acknowledges the critical advice given during the preparation of this paper by his colleagues at KCL, Peter Nash and Warwick Brown, as well as Dr Andrew Lambert at KCL, Jock Gardiner of the Naval Historical Branch, Richard Compton-Hall, former Director of the RN Submarine Museum, Professor Mike Palmer at East Carolina University, Peter Evans of DERA Bincleaves, Rear Admiral J. H. Adams, CB, LVO, and especially Lieutenant Commander Douglas McLean, MA, CD, Dr Roger Sarty, Dr Alec Douglas and Professor Marc Milner in Canada. Any errors of fact, interpretation and balance that remain are, of course, the author's responsibility.

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It cannot be said of many ships of any nationality that they have had two books and a film produced as a result of their efforts. Only one submarine could ever have a signal sent to her reading Hymn No. 30: Verse Five. If you bother to read Hymn 30 in the English Hymnal you will find that verse five reads, "Thus spake the Seraph and, forthwith appeared a shining throng." Clearly we write about the famous HMS Seraph, Barrow-built and still preserved, in part in the United States.

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